The present invention relates to a process for the manufacture of hydrogen cyanide (HCN) and apparatus for use in such a process.
Conventionally, HCN is produced by the so-called Andrussow process, as described in U.S. Pat. Nos. 1,934,838, 4,107,278 and 4,128,622, and in which process ammonia and methane are combusted in air over a platinum-group metal catalyst to produce an effluent stream containing HCN. As a consequence of using air as the source of oxygen, the combustion is inevitably performed in the presence of a large volume of inert nitrogen. This large volume of nitrogen necessitates the use of appropriately sized air compressors and downstream equipment. Additionally, because of the presence of the inert nitrogen, more methane is required to be combusted than that needed merely to raise the temperature of the reactants to a temperature at which the HCN reaction can be sustained over the catalyst. Furthermore, the effluent gas which contains the HCN also contains by-product hydrogen and water, and residual ammonia. However, after separation of the HCN and recoverable ammonia from the other gaseous components, the presence of the inert nitrogen renders the residual gaseous stream of such low fuel value that it is requires its own dedicated burner.
For optimum conditions, the Andrussow process is operated within the flammable limits of the ammonia and methane mixture. The use of oxygen enriched air moves the process closer to the detonable region which makes operation extremely hazardous and as such is conventionally prohibitively difficult to control.
In the present invention, the process is operated such that potentially detonable mixtures of reactants are formed but in such a manner that detonation is avoided. This gives rise to improvements in the energy efficiency of the process and provides an effluent gas stream which has a significantly higher hydrogen content than that obtained from conventional Andrussow processes.
Accordingly, in a first aspect the present invention provides a catalytic process for the manufacture of hydrogen cyanide, which process comprises
(a) forming
(i) an oxygen rich oxidant stream
(ii) at least one oxidant-free feed stream supplying methane and ammonia;
(b) separately preheating by indirect heat exchange at least one of said oxidant and feed streams to a respective oxidant and feed preheat temperature;
(c) rapidly mixing sufficient of the oxidant and feed streams at their respective preheat temperatures in a mixing zone to form a detonable mixed stream at a mixed temperature and which mixed temperature is at least 50xc2x0 C. below the autoignition temperature of the mixed stream;
(d) conveying the mixed stream through the mixing zone at a mixing velocity such that detonation of the mixed stream is avoided; and thereafter
(e) feeding the mixed stream to a catalyst capable of catalysing the formation of hydrogen cyanide from the mixed stream at the mixed temperature to form an effluent stream containing hydrogen cyanide.
In a second aspect the present invention provides an apparatus for use in the process of the first aspect of the invention, which apparatus comprises
(a) a first inlet for an oxygen rich oxidant stream;
(b) at least one second inlet for at least one oxidant-free feed stream supplying methane and ammonia;
(c) a first conduit connected to said first inlet and along which the oxidant stream is able to flow from said first inlet to a discharge end of the first conduit;
(d) at least one second conduit connected to said at least one second inlet and along which the at least one feed stream is able to flow from the at least one second inlet to a discharge end of said at least one second conduit and which discharge end is approximately coterminous with the discharge end of said first conduit;
(e) a mixing zone located at the discharge end of the first conduit for receiving the oxidant and feed streams;
(f) a mixing means located in the mixing zone for effecting rapid mixing of the oxidant and feed streams to form a detonable mixed stream and for conveying the mixed stream through the mixing zone at a mixing velocity such that detonation of the mixed stream is avoided;
(g) a discharge orifice providing a flow connecting means between the mixing zone and a deflagration arrestor and through which the mixed stream is able to flow, the deflagration arrestor capable of inhibiting the propagation of a deflagration of the mixed stream back through the discharge orifice into the mixing zone; and
(h) a reaction zone for receiving the mixed stream from the deflagration arrestor and for directing the mixed stream to a supported catalyst located in the reaction zone for promoting the formation of hydrogen cyanide from the detonable mixed stream;
(i) indirect heat exchange means for preheating at least one of said oxidant and feed streams prior to mixing.
The oxygen rich oxidant stream typically contains from 30 to 100% by volume of oxygen. Preferably, the oxidant stream contains from 50 to 100% by volume of oxygen and in particular from 80 to 100% by volume.
The at least one oxygen-free feed stream may provide the methane and ammonia as separate feed streams which are then separately mixed with the oxygen rich oxidant stream. Preferably, the oxidant free feed stream is a premixed stream containing a mixture of methane and ammonia. Suitably, the volume (and hence molar) ratio of ammonia to methane used in the present process is from 1:1 to 1:1.5, preferably from 1:1 to 1:1.3 and particularly from 1:1 to 1:1.2.
The oxidant and feed streams may contain other components, for example a feed stream containing methane and ammonia may also contain a small proportion of oxygen provided that the composition of the feed stream is outside the detonable region.
In addition to the use of an oxygen rich oxidant stream, the need to use a significant excess of methane is further avoided in the present invention by indirectly preheating at least one of the oxidant and feed streams to a preheat temperature such that when the streams are mixed and passed over a catalyst capable of catalysing the formation of hydrogen cyanide the reaction is sustained and proceeds at a desired catalyst temperature. Where the oxidant stream is preheated it is advisable to avoid such temperatures that could give rise to metallurgical problems. Preferably, the oxidant stream is preheated to a temperature in the range from 200 to 300xc2x0 C. and the at least one feed stream is preheated to a temperature in the range from 300 to 450xc2x0 C. The preheat temperatures are preferably chosen such that the mixed temperature from 200 to 400xc2x0 C., preferably from 300 to 430xc2x0 C. and particularly from 330 to 430xc2x0 C. is achieved. Use of such reaction temperatures normally results in a catalyst temperature of from 1000 to 1250xc2x0 C.
The effluent stream exit the catalyst is approximately at the catalyst temperature and thus represents a valuable source of high grade energy. Consequently, it is preferred that the effluent stream on exiting the catalyst is used in indirect heat exchange to raise useful high pressure steam and hence to provide a partially cooled effluent stream, typically at a temperature from 500 to 700xc2x0 C., e.g. about 600xc2x0 C. The partially cooled effluent stream may also be usefully employed in other indirect heat exchange stages and in particular may be used to preheat at least one of the oxidant and feed streams and hence to provide a cooled effluent stream, typically at a temperature from 200 to 400xc2x0 C., e.g. 300xc2x0 C.
Typically, the effluent stream contains from 15 to 20% by volume of HCN and from 30 to 40% by volume of hydrogen.
The preheating of at least one of the oxidant and feed streams may be performed by one or more separate preheating stages such that the streams are at least partially preheated on entry to the respective first and second conduits. Additionally or alternatively, the preheating may be performed whilst the streams flow along the conduits. Preferably, the preheating is performed by indirect heat exchange with the partially cooled effluent stream whilst the streams are flowing along the conduits.
Preferably, when each of the at least one oxidant free feed streams contain ammonia and methane, each first conduit is associated with and located within a second conduit. This simplifies the construction of the apparatus including the selection of the materials of construction. Particularly preferred is where each first conduit is associated with and located within a second conduit and the partially cooled effluent stream is in indirect heat exchange with the feed stream as the feed stream flows along the second conduit. In this particularly preferred situation the length of the second conduit is dependent on the required preheat temperature to be achieved; in the case where the feed stream is preheated prior to entry into the second conduit then the length of the second conduit may be shorter than in the case where the same preheat temperature is achieved solely within the second conduit. However, in any event, the maximum temperature to which the feed stream is preheated should be less than the autoignition temperature of the methane and ammonia mixture.
The supported catalyst bed may be formed from materials conventionally used to promote the formation of hydrogen cyanide from oxygen, methane and ammonia, e.g. platinum-group metal catalysts. Preferably, a sintered metal or ceramic flame trap is positioned before the catalyst bed in order to inhibit the propagation of any undesirable flame fronts from the catalyst bed to the deflagration arrestors.